Interdepartmental Co-operation
by Dave-ck
Summary: "Why not?" Castle says, blue eyes dancing with excitement. He waves at Lucifer even as he, somehow, manages to continue scribbling, "I mean, look at this guy! I could do so many things with him!" 'Couldn't you just,' Lucifer contemplates, eyeing the man anew. [Caskett in L.A., hot on the heels of Royce's killer, run afoul of Detective Decker and her partner, the Devil.]


A cross over? I know. What am I thinking? Season three of Castle predates Lucifer by a number of years - but let's all just pretend it doesn't, shall we?

No Lucifer spoilers because someone still has not caught up.

––

Lucifer narrows his eyes, pinning the attractive brunette to her seat with nothing but his stare.

"Tell me, Detective, what do you desire?"

"I-" She's fighting it, yes, the internal battle of wills reflected in her eyes. "I want-"

"Yes?" Oh, but she is _deliciously_ complicated, this one. He can taste the hint of her desire locked as it is behind the high walls she has it shut behind.

Lucifer can wait her out, of course, but for the sake of efficiency he asks again, "Come now, darling, what makes that heart thrum in your lovely chest, hmm? What is it you want most out of this world?"

Her eyes narrow. Sharpen. Clarity sparks, just as it does for them all, as the world suddenly – if only momentarily – makes so much more sense. She opens her mouth, yearning in her exhale as she tells him, "Justice."

"Aha!" Lucifer crows, victorious. He does so love it when the virtuous fall, after all. "And _that's_ why you killed him, is it?"

Her head snaps back, the magic spell broken, as she runs her a hand through her frazzled hair, soft-looking loose curls tumbling behind her shoulders. "What?"

"There's no point denying it," Lucifer announces, waving his hands between the odd couple seated in front of him. "We _caught_ you at the scene of the crime."

The dark haired man seated opposite him – Richard something-or-other – looks at his partner. "Beckett, is this guy for real?"

"Very real, I assure you, Ricky-boy."

If the other man is off-put by the nickname, he doesn't show it.

The brunette detective – Katherine Beckett, NYPD, Lucifer knows – scoffs, her eyes almost rolling as she turns to her partner in crime and mutters something about _Los Angeles and sub-par police work_.

Rich, coming from someone caught mid-felony.

"Pardon me, Katherine, but I do believe it's ill-mannered to mumble."

She looks surprised he'd heard her and then contrite. Louder, she says, "Nobody is dead, Mr. Morningstar. Not here, at least."

No murder?

"What in the name of my Father am I doing here, then?"

Trespassing? Theft? The department doesn't call upon it's _highly valued_ civilian consultant for something this – this _trivial_. The Detective knows that Lucifer doesn't bother himself with mundane crimes like _robberies._

(Well. Except for that one time, of course, but that was the exception, not the rule.)

"Who even _are_ you?" Castle asks.

Lucifer grants him an enigmatic smile. "The Devil, of course."

And his reaction is not what Lucifer has come to expect – Fear. Disbelief. Even, occasionally, delight. Those he is accustomed to. But no, the other man's eyes alight with something far worse – fascination.

"Oh, I get it, _Lucifer Morningstar_ , huh? Seems a bit much for a cop to adopt such a unique moniker but – hey, this is Hollywood, right? Tell me, how's that working out for you? Does it help scare suspects into a confession?"

Ignoring the first half of the man's rambling and the other detective's hissed, ' _Castle!'_ , Lucifer replies, "It's not my _name_ that inspires fear."

 _Finally_ , Lucifer finally spots _his_ detective reemerging from the side entrance of the house.

He hopes she has a good explanation for dragging him with her to a _not murder_ , but Chloe simply waves him off when he makes to approach before she stops to talk to a uniformed officer in hushed whispers. Disheartened, he drops back down in his seat and contends himself with New York City's finest.

No. Lucifer is _not_ happy.

He turns his attention back to the couple in front of him, only to find the other man furiously scribbling in a notepad.

From across the table he can make out a hastily sketched 'Cop' even though it's crossed through. Curious, the Devil leans over and reads upside down-

' _Morningstar?_

 _?Mythology buff. ?Identity crisis. ?Escape. ?Prodigal son._

 _Troubled childhood.'_

He'd underlined the last bit. What even-

Murder or not, this is not how Lucifer's interrogations ordinarily go.

"Castle!" The woman appears flustered. "This is _not_ going into a book."

"Why not?" the man says, blue eyes dancing with excitement. He waves at Lucifer even as he, somehow, manages to continue scribbling, "I mean, look at this guy! I could do so many things with him!"

 _Couldn't you just_ , Lucifer contemplates, eyeing the man anew. But the other man _is_ a suspect and his detective would have his head, he's sure, so – nothing else for it, then.

Lucifer snatches the pad from the man and tears out the page, ignoring Castle's protesting, "Hey!"

He folds the sheet carefully and tucks it into his breast pocket. "I thought you were police officers?"

"I am," Beckett says before she indicates her accomplice. "He's a writer."

"A writer?"

Castle nods and Lucifer considers him in his dark jacket and rumpled button down shirt. A detective and a – what? "Journalist?"

"Novelist," Castle corrects and Lucifer snorts.

No, he thinks not. "There is enough _bad_ _fiction_ blaspheming my name, thank you."

The man looks offended at the descriptor and it delights something within Lucifer, brightening his altogether dreary day.

"You're not a cop, either," the writer declares, a previously concealed spark of intelligence surging in his blue eyes. "Too well dressed, for one thing."

Well. This may be amusing, at least.

Perhaps he can have some fun whilst his detective helps out her colleagues in Robbery after all?

"Go on," Lucifer says, waving his hand in invitation. "You're off to such a good start."

Castle settles back, looking Lucifer over with an assessing gaze.

"You're intelligent. Educated. Ivy League, maybe?" At Lucifer's head shake, the other man amends, "Very well read, at least. I'd bet that you're an attorney, except that you obviously don't care very much about police procedure."

Lucifer lets his gaze slide over to the other detective during her partner's assessment. Underneath her attempt to appear bored, Beckett is becoming agitated, he can tell, eager to move on to whatever _Justice_ she is seeking.

Maybe, Lucifer muses, that's why Chloe is still keeping her distance? Trying to keep the other woman on her toes, perhaps? Always good at playing the long game, his detective.

"So you're definitely not a cop," Castle summates, interrupting Lucifer's train of thought.

"Of course not," Lucifer acknowledges at last. "I am far too important for police work."

From the corner of his eye he notices Chloe shoot him a quick, annoyed look from where she's standing – aha, so she is paying attention after all – and Lucifer grins. He finds himself rather proud he's managed to ruffle her in some way.

A slightly less annoyed look is etched on the face of the detective in front of him as she asks, "What do you call this, then?"

Lucifer focuses his full attention back on the couple and away from his erstwhile partner.

"I dabble," Lucifer admits, "when the department is in dire need of my assistance."

The writer – Richard Castle? Yes, okay, maybe that name _is_ familiar now that he thinks about it – is still studying him. Lucifer might have found it uncomfortable if he wasn't so used to the attention.

"So," he says at last. "A detective and a novelist walk into a bar-"

"Owned by the _devil_ ," Castle interjects. He turns to his partner. "This is the guy who owns that nightclub I was telling you about." He turns back to Lucifer. "Right?"

Lucifer nods. "I'd invite you as my guests, but I'm not terribly fond of having thieves on my property."

"A nightclub owner?" Beckett asks at the same time her partner declares, "We didn't steal anything!"

Lucifer nods to the detective while he concedes, "Trespassers, then."

"We're investigating-" the writer starts before his partner's sharp elbow catches him in the side.

Intriguing.

Beckett leans forward, trying to engage him, but Lucifer is more than familiar with _Interrogation 1.01_. "Why is a nightclub owner working with the police?"

At Lucifer's raised eyebrow the detective adds, "You have to admit it's an odd match."

"You're one to talk," Lucifer points out, allowing her the distraction for now. "And how did _your_ partnership come about, exactly, hmm?"

Lucifer's eyes dart between the two of them, assessing, and – yes. Something flickers in the novelist's eyes. Of course. Lucifer can sense it now.

"You," he says, addressing the writer. "You wanted something from her. You _still_ do."

The other man's back straightens and Lucifer's grin widens. Oh, how he loves finding the end of a thread and tugging. "What is it you wanted? Sex?

"No, that's not it." The man is handsome enough and at least moderately successful in his field. He probably has ample opportunity to bed sexual partners. No, what he wants is – "Something decidedly less carnal and altogether _more_."

"You don't have to answer that, Castle," Beckett interjects at her partner's obvious discomfort. "What kind of a sick game are you playing?"

Lucifer ignores her.

"Look at me," he entices when the other man's attention wavers and blue eyes snap back to his immediately. "Tell me, Richard, what secret lies buried deep in that yearning soul of yours?"

Castle's eyes glaze, his mouth drops open and Lucifer can almost taste the answer in the air between them. _Almost._ The devil grasps the thread of desire and gives it a firm tug. "What is it you _crave_ from your detective?"

"One and done," Castle finally exclaims in one quick breath.

One and done?

Lucifer sits back and releases his hold on the other man. The answer means absolutely nothing to him but it must mean a great deal of _something_ to the pair in front of him as they both immediately flush pink.

Beckett's gaze snaps to her partner as he looks anywhere but at her.

Interesting.

Lucifer's about to prod further when Chloe seats herself on the bench next to him.

"Chloe Decker, LAPD," she says by way of introduction.

Beckett's jaw snaps shut with a click of teeth as she turns to face the proverbial music. She is, to Lucifer's utter enjoyment, still pink.

"Are you going to arrest us, Detective Decker?"

"Can you give me one reason not to?"

"We're here looking into a friend's murder," Beckett admits, still a little too flustered to lie.

"Doesn't give you the right to break into someone's home," Lucifer interjects.

The NYPD detective and her partner offer no defence.

Chloe looks at him and Lucifer shrugs. He didn't mean to break them – and he's still not entirely sure how ' _One and done'_ even managed to do so.

"Consider yourself lucky I don't have the time to waste on the paperwork." Chloe says at last. She waits a beat before continuing, "No. Instead, I have someone on the line for you, Detective Beckett."

Chloe drops her phone on the table between them. "Sir, you're on speaker."

a/n: Apologies for any mistakes - this isn't Beta'd and I had more trouble jumping between Castle and Lucifer head spaces than I anticipated. Kudos to those who actually *can* write cross overs.


End file.
